1. Field of the Invention
The present invention relates generally to epoxy resins and particularly to epoxy resins that are toughened with thermoplastic materials. Thermoplastic-toughened epoxy resins are used to make high performance composite parts. More particularly, the present invention is directed to increasing the out-time or shelf-life of epoxy resins when they are stored at room temperature without adversely affecting the cure rate of the resins when they are cured at conventional curing temperatures.
2. Description of Related Art
Composite materials are typically composed of a resin matrix and reinforcing fibers as the two primary constituents. Resin matrices that contain one or more epoxy resins as a principal ingredient are widely used. The composite materials are often required to perform in demanding environments, such as in the field of aerospace where the physical limits and characteristics of composite parts are of critical importance.
Pre-impregnated composite material (prepreg is used widely in the manufacture of composite parts. Prepreg is a combination of uncured resin and fiber reinforcement, which is in a form that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and insure that the resin is distributed in the network as desired. It is well known that the relative amount of fibers and resin in a composite part and the distribution of resin within the fiber network have a large effect on the structural properties of the part. Prepreg is a preferred material for use in manufacturing load-bearing or structural parts and particularly aerospace structural parts, such as wings, fuselages, bulkheads and control surfaces. It is important that these parts have sufficient strength, damage tolerance, interlaminar fracture toughness and other requirements that are routinely established for such parts.
The fiber reinforcements that are commonly used in aerospace prepreg are multidirectional woven fabrics or unidirectional tape that contains fibers extending parallel to each other. The fibers are typically in the form of bundles of numerous individual fibers or filaments that are referred to as a “tows”. The fibers or tows can also be chopped and randomly oriented in the resin to form a non-woven mat. These various fiber reinforcement configurations are impregnated with a carefully controlled amount of uncured resin. The resulting prepreg is typically placed between protective layers and rolled up for storage or transport to the manufacturing facility.
Prepreg may also be in the form of short segments of chopped unidirectional tape that are randomly oriented to form a non-woven mat of chopped unidirectional tape. This type of prepreg is referred to as a “quasi-isotropic chopped” prepreg. Quasi-isotropic chopped prepreg is similar to the more traditional non-woven fiber mat prepreg, except that short lengths of chopped unidirectional tape (chips) are randomly oriented in the mat rather than chopped fibers.
The tensile strength of a cured composite material is largely dictated by the individual properties of the reinforcing fiber and matrix resin, as well as the interaction between these two components. In addition, the fiber-resin volume ratio is an important factor. Cured composites that are under tension tend to fail through a mechanism of accumulated damage arising from multiple tensile breakages of the individual fiber filaments located in the reinforcement tows. Once the stress levels in the resin adjacent to the broken filament ends becomes too great, the whole composite can fail. Therefore, fiber strength, the strength of the resin matrix, and the efficiency of stress dissipation in the vicinity of broken filament ends all contribute to the tensile strength of a cured composite material.
In many applications, it is desirable to maximize the tensile strength property of the cured composite material. However, attempts to maximize tensile strength can often result in negative effects on other desirable properties, such as the compression performance and damage tolerance. In addition, attempts to maximize tensile strength can have unpredictable effects on the viscosity, tack and out-time of the resin matrix.
One method of increasing composite tensile performance and resistance to damage is to include one or more thermoplastic materials in the epoxy resin matrix. A variety of different thermoplastic materials in a variety of different forms have been used to toughen epoxy resins. For example, see U.S. Pat. No. 7,754,322.
Multiple layers of prepreg are commonly used to form composite parts for structural applications that have a laminated structure. Delamination of such composite parts is also a possible failure mode. Delamination occurs when two layers de-bond from each other. Important design limiting factors include both the energy needed to initiate a delamination and the energy needed to propagate it. The initiation and growth of a delamination is often determined by examining Mode I and Mode II fracture toughness. Fracture toughness is usually measured using composite materials that have a unidirectional fiber orientation. The interlaminar fracture toughness of a composite material is quantified using the G1c (Double Cantilever Beam) and G2c (End Notch Flex) tests. In Mode I, the pre-cracked laminate failure is governed by peel forces and in Mode II the crack is propagated by shear forces. The G2c interlaminar fracture toughness is related to the laminates ability to compress when impacted. This compressive property is measured as the compression of the laminate after a designated impact (CAI). Prepreg materials that exhibit high damage tolerances also tend have high CAI and G2c values.
The viscosity of the uncured resin is an important factor that must be taken into consideration when forming prepreg or when the resin is used in a molding process. The viscosity of the resin must be low enough to insure that the resin components can be mixed completely and then impregnated thoroughly into the reinforcing fibers. The viscosity of the resin must also be high enough to insure that the resin does not flow to any substantial degree during storage or lay-up of the prepreg. Resins that do not have viscosities which meet these basic requirements cannot be used to make prepreg. The viscosity of the uncured resin must remain within acceptable limits during storage in order for the cured composite part to exhibit desired levels of strength and/or damage tolerance.
The stickiness or tackiness of the uncured prepreg is commonly referred to as “tack”. The tack of uncured prepreg is an important consideration during lay-up and molding operations. Prepreg with little or no tack is difficult to form into laminates that can be molded to form composite parts. Conversely, prepreg with too much tack can be difficult to handle and also difficult to place into the mold. It is desirable that the prepreg have the right amount of tack to insure easy handling and good laminate/molding characteristics. It is important that the tack of the uncured resin and prepreg remain within acceptable limits during storage and handling to insure that desired levels of strength and/or damage tolerance can be obtained for a given cured composite.
The “out-time” or “shelf-life” of uncured resin is the length of time that the resin may be exposed to ambient conditions before undergoing an unacceptable degree of curing which can adversely affect important resin properties, such as viscosity and tack. The out-time of epoxy resin at room temperature can vary widely depending upon a variety of factors, but is principally controlled by the resin formulation being used and particularly by the types and amounts of curative agents that are included in the resin. The resin out-time must be sufficiently long to allow storage, transport, normal handling, lay-up and molding operations to be accomplished without the resin undergoing unacceptable levels of curing.
The amounts and types of curative agents must also be such that the uncured resin can be cured according to the curing processes that are typically used to make thermoplastic toughened epoxy composite parts. Typical curing processes for thermoplastic-toughened epoxy resins that are used to make structural parts involve heating under pressure at a temperature of between 175° C. and 185° C. for at least two hours. This requirement that the uncured resin exhibit suitable curing properties at conventional curing temperatures has a direct effect on ones ability to extend room temperature out-time of the uncured resin. In general, the current thermoplastic toughened epoxy resins, which are cured at about 177° C., have a shelf-life at room temperature of a maximum of 2 to 3 weeks.
It would be desirable to provide a thermoplastic toughened epoxy resin that exhibits all of the structural properties that are expected from such toughened resins systems, but which can be stored at room temperature for periods of up to 6 weeks or more and then cured under conventional curing conditions for such thermoplastic toughened epoxy resins.